> Bedrock now uses the lexical-binding: t cookie in all its .el files.

This is the right thing to do if you wrote your .el files recently and assumed lexical-binding was true and you understand the implications thereof.

OTOH if your .el files are older and written before lexical binding was possible, it's best to set the cookies of those files to nil. Or go over them with a fine-toothed comb and rewrite them (if necessary) for lexical binding.