Feats are also an optional rule, but I’ve never heard of a table not using them. “Optional rule” in 5e is kind of like the term “theory” IRL, in that some really are optional and some are basically always used. I will admit that not all tables use the diagonal rule, though.
That is only listed in 5e as an optional rule, by default a square is a square and is 5ft regardless of diagonal or not.
Feats are also an optional rule, but I’ve never heard of a table not using them. “Optional rule” in 5e is kind of like the term “theory” IRL, in that some really are optional and some are basically always used. I will admit that not all tables use the diagonal rule, though.