DB-25 is the standard RS-232 connector. DE9 is just the cheap alternative for cases where you don't need all control lines

Doesn't a DB9 connector include all the DB25 RS-232 handshaking lines, even if not all devices actually use them?

I grew up in the 70s-80s with serial connectors and a drawer full of cables, DB25-DB9 adaptors, gender-benders, null modems, breakout boxes, etc, and the only (very common) source of incompatibility that I can recall was connecting devices where one side wanted hardware handshaking but the other didn't provide it, so having to make custom cables with handshaking tied hi/lo to fake it.

Some devices used software XON/XOFF handshaking, so for example on a typical terminal, depending on what you were connected to, you could pause text being sent to the terminal with XOFF (Ctrl-Q), and resume with XON (Ctrl-S).

I've got a softspot for serial communications - used be more a source of fun rather than frustration to dip into the draw of cables/etc and get two devices happily talking to each other.