21. Alphuck, 676 bytes

Surprisingly, most of the code is not padding.

.11111111"1"11111111.
?|+->Hi, Retina!opaos
iipiiciasceaecppisaic
sapiceasccpisipiiiiia
ecsaiiijaeepiiiiiiaec
saeeejeepiiiaeecsajee
eeeepiaeecsaejipiiiii
iaecsaijeeeeeeeeeejii
iiiijiipiiiaecsaijiii
piaeeeecsaijeejiiijii
iiiiiiiiiiijiipiiiaec
saijiipiaeeeecsaejiii
iiiiiiijeeeeeejiiijpi
iaeeeeecsaeeejpiiaeee
eeeecsajeejiiijiiiiii
iijeeeeeeeeeeejeeepia
eeecsaeejeeeeeeeeeeee
jpiaeeeeecsaijepiaeee
csaeejeeeeeeeeejiiiii
iiiiijiipiiiaecsaiiij
epiiiiaecsaeeejiipiae
eeecsaijepiaeeecsaeje
eeeeeeeeeejiiiiiiiiii
iijiipiiiaecsaiijpiae
eecsaejipiaeeecsajiii
piaeeeecsajiiiiiiiiii
ijeeejiiiiiiiijejiipi
iiaecsajpHenry Jams?a
bcefghiwklmnopqrstuvw
xyzabcdefghwuklmnopqr
stuvwxyzabcdefg~

Try it online!

Satisfies:

2. The first character is a ..
3. It contains an e.
4. Its length is even.
5. Its length is a perfect square.
6. It contains an a.
7. It contains a > character.
8. Contains the exact string ->.
9. Contains the exact string Hi, Retina!.
10. The sum of the first two Unicode code points is a multiple of 5.
11. The 10-th character is a ".
12. The last non-empty line does not have any duplicate characters.
13. The first line is a palindrome of length > 5.
14. The first line is exactly 21 characters long (not including newline).
15. It contains a ?.
16. It contains a |.
17. Contains a +.
18. It is at least 28 lines long.
19. The following characters are used five times in total: !"#$.[\] and the codepoint of the second character is less than 60.
20. Contains Henry Jams? as a continuous substring.
21. The last character is ~.

For future answers:

• The first character is a ., and so is the 21st character (palindromic rule).
• The 10th character is a ", and so is the 12th character (palindromic rule).
• The first line is a palindrome of length 21.
• The second character's Unicode code point, mod 5, is 4, and its code point is lower than 60 (the printables are ',1;6 and tab).
• The last character is ~.

• Its length is an even perfect square.
• It is at least 28 lines long.
• The last non-empty line does not have any duplicate characters.

• Contains the exact sequence ->.
• Contains the exact strings Hi, Retina! and Henry Jams?.
• It contains ?, | and +.

• Each program is now allowed only the 2 . and 2 " in the first line, and the ! in Hi, Retina!. Those characters cannot be used anywhere else, in addition to no uses of #$[\].

21. Alphuck, 676 bytes

Surprisingly, most of the code is not padding.

.11111111"1"11111111.
?|+->Hi, Retina!opaos
iipiiciasceaecppisaic
sapiceasccpisipiiiiia
ecsaiiijaeepiiiiiiaec
saeeejeepiiiaeecsajee
eeeepiaeecsaejipiiiii
iaecsaijeeeeeeeeeejii
iiiijiipiiiaecsaijiii
piaeeeecsaijeejiiijii
iiiiiiiiiiijiipiiiaec
saijiipiaeeeecsaejiii
iiiiiiijeeeeeejiiijpi
iaeeeeecsaeeejpiiaeee
eeeecsajeejiiijiiiiii
iijeeeeeeeeeeejeeepia
eeecsaeejeeeeeeeeeeee
jpiaeeeeecsaijepiaeee
csaeejeeeeeeeeejiiiii
iiiiijiipiiiaecsaiiij
epiiiiaecsaeeejiipiae
eeecsaijepiaeeecsaeje
eeeeeeeeeejiiiiiiiiii
iijiipiiiaecsaiijpiae
eecsaejipiaeeecsajiii
piaeeeecsajiiiiiiiiii
ijeeejiiiiiiiijejiipi
iiaecsajpHenry Jams?a
bcefghiwklmnopqrstuvw
xyzabcdefghwuklmnopqr
stuvwxyzabcdefg~

Try it online!

Satisfies:

2. The first character is a ..
3. It contains an e.
4. Its length is even.
5. Its length is a perfect square.
6. It contains an a.
7. It contains a > character.
8. Contains the exact string ->.
9. Contains the exact string Hi, Retina!.
10. The sum of the first two Unicode code points is a multiple of 5.
11. The 10-th character is a ".
12. The last non-empty line does not have any duplicate characters.
13. The first line is a palindrome of length > 5.
14. The first line is exactly 21 characters long (not including newline).
15. It contains a ?.
16. It contains a |.
17. Contains a +.
18. It is at least 28 lines long.
19. The following characters are used five times in total: !"#$.[\] and the codepoint of the second character is less than 60.
20. Contains Henry Jams? as a continuous substring.
21. The last character is ~.

For future answers:

• The first character is a ., and so is the 21st character (palindromic rule).
• The 10th character is a ", and so is the 12th character (palindromic rule).
• The first line is a palindrome of length 21.
• The second character's Unicode code point, mod 5, is 4, and its code point is lower than 60 (the printables are ',1;6 and tab).
• The last character is ~.

• Its length is an even perfect square.
• It is at least 28 lines long.
• The last non-empty line does not have any duplicate characters.

• Contains the exact sequence ->.
• Contains the exact strings Hi, Retina! and Henry Jams?.
• It contains | and +.

• Each program is now allowed only the 2 . and 2 " in the first line, and the ! in Hi, Retina!. Those characters cannot be used anywhere else, in addition to no uses of #$[\].

Surprisingly, most of the code is not padding. Or is it?

• The first character is a ., and so is the 21st character (palindromic rule).
• Its length is an even perfect square.
• Contains the exact sequence ->.
• Contains the exact string Hi, Retina!.
• The second character's Unicode code point, mod 5, is 4, and its code point is lower than 60.
• The 10-th character is a ", and so is the twelfth character (palindromic rule).
• The last non-empty line does not have any duplicate characters.
• The first line is a palindrome of length = 21
• It contains a ?, a | and a +.
• It is at least 28 lines long.
• The following characters can only be used five times in total: !"#$.[\].
• Each program is now allowed only the 2 . and 2 " in the first line, and the ! in Hi, Retina!. Those characters cannot be used anywhere else, in addition to no uses of #$[\].
• Each program must contain Henry Jams? as a continuous substring.
• The last character is ~.

Surprisingly, most of the code is not padding.

• The first character is a ., and so is the 21st character (palindromic rule).
• The 10th character is a ", and so is the 12th character (palindromic rule).
• The first line is a palindrome of length 21.
• The second character's Unicode code point, mod 5, is 4, and its code point is lower than 60 (the printables are ',1;6 and tab).
• The last character is ~.

• Its length is an even perfect square.
• It is at least 28 lines long.
• The last non-empty line does not have any duplicate characters.
 

• Contains the exact sequence ->.
• Contains the exact strings Hi, Retina! and Henry Jams?.
• It contains ?, | and +.
 

• Each program is now allowed only the 2 . and 2 " in the first line, and the ! in Hi, Retina!. Those characters cannot be used anywhere else, in addition to no uses of #$[\].