So this just looks like the Community has to take care of the projects and/or find alternatives

Alternatives

File manager - Material Files

That one doesnt have the security vulnerability and I think it is also better. But they are pretty much equal.

Simple Gallery - Aves

I prefer Simple Gallery though, it is pretty great

Simple dialer - Welefon

Very basic, I personally use ACR phone for call recording, with disabled internet permissions. The Dev asked on Reddit about opensourcing the App and got negative responses, so maybe unlikely.

Simple Keyboard - Florisboard

Currently unmaintained but awesome. I hope it gets revived.

Simple Calculator

My favourite: Mint Calculator

Calculator

yetCalc

Unitto

Simple SMS

AOSP SMS/MMS, Quik Sms

Deku SMS supports encryption. This is not a replacement for secure messengers, metadata is still a mess, but it is pretty cool.

AOSP SMS is poorly hard to find. That would be great on F-Droid. It also uses old libraries.

All these look way better and have unit transformations.

Some Gems

Some apps have no good replacement

Simple Gallery

Also in my opinion just the best. The Amaze Videoplayer and tools are great though.

Simple Contacts SE

That is the better version with some enhanced features.

Simple Calendar

I just got way better results than using Etar. The Widget is great too.