"1. The old icons were no longer up to date; many actions did not have an (appropriate) icon."

Icons dont degrade ( unless the supported format changes) so the "up to date' is a 'change to a new (icon) fashion'
and rather than add an appropriate icon you made an entire new (worse) icon set. Very good.
(Note it's worse because smaller, low contrast and no color)

"2. They were not adapted to high resolution displays (and the pie menu looked awful on those)."
Presumably the new ones are now adapted to high resolution displays but now look like uncontrasted random scribble everywhere (else) - also excellent

"3. Their sources were not available, so I could not update them."

Because of course the only way to update *icons* is from their source.

"4. Their license was unknown."
But historically released and freely available with this use .

"If you provide me with a new set of icons satisfying the above requirements, I might consider providing an option to use them."

Rather than just providing an option to use the original icons where they might make more sense to do so...

Conversely if you provide an option to use the old icons I might consider continuing to use this add-on. Otherwise I guess a downgrade and update halt is in order - the latest update has nothing i need or desire and actually makes the
usability, recognition and functionality of the add-on worse rather than better...

Thx for your efforts tho'