I don't like how companies behave like that and basically push users to upgrade their phones
Garmin in particular makes it mandatory to use their app for SOME connected functionalities (while others work just fine on wifi or wifi tethering).
They unsupported old version of android for the garmin connect app pretty fast (my mom's phone was incompatible within 4 years of its release) while they don't support you to connect older devices on newer phones and say they know it doesn't work.
As a user, I don't care whose fault it is.
I ditched both Google in favour of degooglized android on older Xiaomi and Pixel phones that support custom ROMs, and Garmin for any sport equipment.
My next phone will be a Fairphone if they make something with a smaller screen.
I don't know which app you're doing, but I would most likely permanently just not download it or find an open source alternative if it stopped working for me, as no app is essential.
Pay attention to the user-base, in particular is your app is supposed to work with a web of users.