Aborting evaluation usign menu Evaluation -> Abort Evaluation, or equivalently 'Alt + .' key shortcut doesn't work in my Mathematica 18.104.22.168. Is it a known bug?
I can confirm this issue.
For example, the following code will hang the front end of v12.0.0.
(* !!! SAVE ALL YOUR WORK BEFORE RUNNING THIS !!! *) For[i = 0, True, i++, Print[i]]
As noted by Lucas in comments, one can use Task Manager/Activity Monitor to terminate WolframKernel process (but not Mathematica process). This way one will loose all evaluation results, but has a chance to un-hang the front end and save the notebook.