For the last several months, everytime your program, which I have been using for over 10 years, tries to update, it uninstalls itself and disappears. I am running Windows 8.1 64 on a standard box and have the professional edition. Is there a workaround to this? Why is this happening only this year?
One workaround for older OSes (I'm not sure if it's Win8.1 specifically, although it has been recommended for Win7) is to instead download and install the Slim builds going forward (something to do with how the installer works now):
https://www.ccleaner.com/ccleaner/builds
Also check your antivirus logs, not too long ago a couple of antivirus' wouldn't allow a new version due to false positives.
If it's being removed after it has installed then that's usually being done by an Antivirus app.
There are currently (at least) three AV's where if the AV is an old version it will remove the latest CCleaner versions.
Updating from within the AV itself will not always cure the issue, you need to download the latest version from the AV's website.
For example - Trend Micro 2018 and 2019 will both say that they are up to date but will remove newer CCleaner versions, Updating Trend Micro from their website to the 2020 version stops that happening.
Which antivirus are you using?