How essential is the 64-bit compatibility for macOS?

NancyC wrote:

I keep getting the message that certain apps are not optimized and will not work with future versions of macOS because of the move to 64-bit compatibility. But why am I, as an end-user, be the one expected to contact the developers to update their apps?? (Sorry, I have Mojave, not High Sierra)

This is a generic warning only. You have to do nothing, 32-bit apps continue to work on Mojave.

In the future release of macOS 10.15, applications will have to be 64-bit. App developers will issue updated apps if they want their apps to work on these future releases.

32-bit app compatibility with macOS High Sierra 10.13 ... - Apple Support

https://support.apple.com/en-us/HT208436

Because you are the one who chooses which third party apps you want. Apple does not know what apps you want to run. If your third party app developers want you to continue using their apps it is up to them to keep their apps compatible and it is up to you to use compatible apps.

Your developers most likely already know, but either they do not have their apps check for updates (so you are not being given the opportunity to upgrade your app), they limit updates to point released in your version and make you buy the next major version (but again have not notified you that a new major version is available), or they are not interested in updating their app for 64-bits.

If your developer has not already notified you, then it is up to you to see if there is a new version assuming you will A) be upgrading your Mac to the next macOS release after Mojave 10.14, _AND_ B) you want to continue using this particular app.

Until you decided to install the next major macOS version after Mojave 10.14, you can ignore these warning. Just know unless these apps are upgraded, they will stop working if you do upgrade your macOS.