Can Apple developers see app uninstalls? I know that Apple developers can find out how many users have downloaded their app(s) from the App Store. But I would like to know if developers can also find out how many users have uninstalled their app(s)?
No, iOS does not send a ping when an app is deleted from the device. Also, user choice determines whether iOS reports generic analytic information to Apple, so even if Apple wanted to process that analytics and possibly report that to a developer, it’s opt in by end user. 

Developers that desire that need to build an activity reporting service that exists outside the App Store. Look at MAU, monthly active users, count of launches, etc...  just be sure you think of the privacy concerns. Once you start collecting this data, you are liable under GDPR and many governments to handle it properly and respond to legal and extra-judicial requests properly. 

Not collecting this data and not retaining it can be a very smart business decision. Measuring engagement can be very valuable, but think how to benefit the end user and consider privacy while you do that. You might request this to be aggregated like the installs are as I could see Apple providing that in the future if it was balanced and useful to most developers rather than forcing them to roll their own analytics.