Sites like APKMirror aren't pirate sites, as they only host APKs that are free of charge. Also, if you're using Android without Play Services but still want to download Play Store apps, you can use the "Yalp Store" app from F-Droid.
I'm not making a moral judgement, maybe 'pirate' was the wrong word. It probably was something like APKMirror, and my point was the chain of trust is broken (you have decide to trust that site and its provenance model) vs an HTTPS download from the Slack domain.
Yalp Store is a great app! Incidentally, it violates the Play Store ToS (this is even written in their FAQ[0])