An ARCore Update for Pokemon GO was put through eight months ago that was supposed to prevent annoying "enable AR+" or "update ARCore" pop ups, though some players still have to deal with them. Here's why they're caused and how to get rid of them.
ARCore Update Pokemon GO: What Is It?
AR Core is the software on Android devices that is used to enable the AR+ mode in Pokemon GO. AR+ being the mode implemented in 2018 that allows for more realistic placement of Pokemon in the environment and registers how close or far away you are from the Pokemon. This is used both when catching Pokemon and playing with buddies, including the new Mega buddies. Some players enjoy the immersion granted through AR+ however other players may dislike the long load times or get motion sickness while using it. ARCore is also not supported on older devices making the popups even more annoying for players who cannot enable the feature.
How To Fix It
If you like playing with AR+ and would just like Pokemon GO to stop hassling you about updates, you can manually update ARCore in Apps and Notifications settings. If you would rather just throw the whole thing out, you can uninstall ARCore in the same menu, just be aware it might effect other AR based apps as well. If you just don't care about AR at all let alone AR + you can also edit Pokemon GO's privileges in settings and turn off camera access.