Currently, src/main/java/org/jabref/logic/bibtexkeypattern/BibtexKeyGenerator.java
states

KEY_ILLEGAL_CHARACTERS = "{}(),\\\"-#~^:'`ʹ";

This list of characters is used to display a warning for the field bibtexkey.

Following issue #5982, I digged a bit further about the incompatible characters in a bibtexkey.
According to https://tex.stackexchange.com/questions/408530/what-characters-are-allowed-to-use-as-delimiters-for-bibtex-keys , for biber, % and = should not be used, and hence be added to the illegal characters.