As it continues the 64-bit app transition, Apple will begin alerting customers running macOS High Sierra 10.13.4 when they are using an outdated, 32-bit app. The alert will only happen when you open a 32-bit app, and it will happen once, Apple told me earlier today.
The alert is a way of communicating to customers that some action needs to be taken, albeit not immediately. Apple said they don’t have a firm timeline of when 32-bit apps will stop working, but they informed developers at last year’s WWDC that macOS High Sierra would be the final version to run 32-bit apps without compromise.
The alert that is shown when you open the app tells people that the developer needs to update the app to make it compatible going forward. You can still use the app in its current form, but at some point in the future, it won’t work.
Apple is taking this step to ensure that the Mac apps we use are as advanced as the Mac itself.
Apple has posted a support article explaining more on the transition to 64-bit, as well as offering a few answers for affected users.
There is no need to panic if you see this alert, but please do take note of it. You should contact the developer of the app to see if they plan to update it and when.