I don't even have to ever click on that, it just expands on mouse-over here in Firefox 70. Is that different for you?

Yes, a click keeps the menu open and the arrow is the indicator for this. The CSS style is still there

.vectorMenuCheckbox:checked + h3 span:after {
	transform: scaleY( -1 );
}

but is doesn't work anymore.

Caused by rSVEC481c90798e53: Make extra `<span>` in Vector tabs optional.

Change 554332 had a related patch set uploaded (by Fomafix; owner: Fomafix):
[mediawiki/skins/Vector@master] Restore arrow flipping for menu indicator

https://gerrit.wikimedia.org/r/554332

Change 554332 merged by jenkins-bot:
[mediawiki/skins/Vector@master] Restore arrow flipping for menu indicator

https://gerrit.wikimedia.org/r/554332