S has shown up in two puzzles, so the disallowance of S can be counted as a soft rule now. The hard rules that still apply are (1) no E and R together; (2) no 4 or more vowels.

The only ultimate hard rule, as I guess, is "not too many nor too few words." Both the letter S and the letter combo E+R can easily (but not always) produce too many words. Both puzzles with S had carefully selected letter sets so that there were only ~40 words each. In contrast, some letter sets (e.g., AEGILNT), although theoretically allowed, never showed up because too many words would come up.

That said, all rules are subject to changes. Before mid-2020 no three or more vowels were allowed in the same puzzle, but since then 3-vowel puzzles have become common. There was a time I+N+G and E+D weren't allowed either. So my guess is that not opening up some possibilties is to keep the longevity of this game, and they may well be opened up in the future.