On iOS, you can connect your device to a computer running iTunes 12.6.3[1] before updating an app to save the older IPA file to your computer. If you later decide to downgrade, you can delete the app and then restore the old IPA from your computer. Backup the IPAs separately from iTunes, as iTunes syncing can screw you if you're not careful.
Yes, this is a complete and total pain in the neck, but it has saved me in the past. Jailbroken devices can also use tweaks that allow downgrading right from the App Store.
P.S. There's actually one more way to downgrade apps—use a version of iTunes from before Apple introduced certificate pinning (iTunes 7 worked for me) and use Charles Proxy to edit the request so Apple gives you an old version. This worked for me as of last fall—but in this case, "pain in the neck" does not even begin to describe it. :)
Yes, this is a complete and total pain in the neck, but it has saved me in the past. Jailbroken devices can also use tweaks that allow downgrading right from the App Store.
---
[1] https://support.apple.com/en-us/HT208079
P.S. There's actually one more way to downgrade apps—use a version of iTunes from before Apple introduced certificate pinning (iTunes 7 worked for me) and use Charles Proxy to edit the request so Apple gives you an old version. This worked for me as of last fall—but in this case, "pain in the neck" does not even begin to describe it. :)