a) 后天下之乐而乐
  b) 後天下之樂而樂
  c) 後天下之楽而楽

a) is clearly Simplified Chinese from a sibling comment, b) is Traditional copied from your comment, and c) is as I just typed in my own language. Unicode Hanzi/Kanji are a mess and there are characters same or different, in appearance or in binary, depending on intended variants, languages, fonts, systems, keyboard, distance between Earth and Alpha Centauri, etc.

Fascinating! That's exactly why I asked, so thank you.

Do people usually recognize all variants as valid and legible? Or does any particular set of letters/symbols prevail in practice?

Very location dependent. But when you learn to write the characters you understand the variants differently. They look like random strokes to an untrained eye. But they’re not. I’m not sure if that makes sense.

Take a lowercase a in English for example. This font writes it differently than a child. Or in cursive. Or probably than you would write it. But you recognize all of them and don’t really think about it.

Traditional kinds are usually recognizable, but I'd be unsure or straight up wrong about most Simplified versions. Overall proportions and small details often feel "wrong" for both as well due to cultures converging at different points.

a) Simplified Chinese

b) Traditional Chinese

c) 楽 is a variation of 樂, which is now widely used in Japanese Kanji but deprecated in Traditional Chinese.

Note:

A variation means some people write 樂 as 楽 in ancient China, but not widely adopted.

Kanji is a Japanese word, means "Chinese Character".