While the concept of "inbox zero" is appealing in theory, many of us just fail to achieve it. Instead, we do our best to archive or label messages, mark unwanted email as spam, and delete some threads when we're completely done with them. If you often use the last two options, you'll find Gmail for Android has introduced a new annoyance in your workflow as it's recently removed the handy option to completely get rid of spam and deleted messages.
In previous versions of Gmail, the Empty trash now and Empty spam now options showed up at the top of the email list when you checked your trash and spam folders, respectively. In the most recent version, these sections are gone. You can still multi-select messages manually, but no permanent delete option is offered after that.
Left: Previous. Right: Now.
In the case of the spam folder, you can still delete messages after selecting them, but that'll just move them to the trash, where they'll have to wait 30 days to be automatically deleted for good.
Left: Previous. Middle & Right: Now.
The only way to completely get rid of all these unwanted messages is to use the web interface for gmail, where the empty option is still present.
We verified this absolutely unnecessary change is present in Gmail v2020.02.02 (APK Mirror), but we're not sure if it was triggered server-side or not. On last month's v2019.12.30 (APK Mirror), the options are still there for me, so you may want to revert to an older version of the app if you rely on them in your email workflow.
It's a bug
This appears to be a display bug with the Gmail app. Simply change your phone's orientation or open any email from the spam/trash then go back to the list and you'll get back the option to empty them. The fix is temporary, so the next time you open those folders, you'll have to do it again, but it's simple enough.