According to https://fishshell.com/docs/4.1/terminal-compatibility (I haven't checked the actual behavior) OSC sequences use an inconsistent terminator, e.g. it's BEL (\x07) for OSC 7, while ST (\e\\) for OSC 8.

The official terminator is ST. This is the only one mentioned in at least ECMA-48, DEC STD 070, and EK-VT520-RM.

BEL, as far as I know, originates from xterm, and was copied by most (if not all) graphical terminals.

It would nice to see consistent usage (incl. docs) in fish, preferably the real official form.