Yes, there are people who say it was added later. I think this is because some older manuscripts translated are said to not have the report about the adulteress.
But > to my knowledge, for you to do what you can to evaluate > just because certain manuscript sources are older, this does not automatically mean they are more reliable. I have understood, also, that the older ones without the adulteress report were found in Egyptian areas and elsewhere, other than from where the ones with the adulteress report were found.
Older ones could survive if the church was not using them and therefore they were not worn out. Younger manuscripts could be the only ones found in a region because older ones had been used and worn out. So, I don't automatically assume older means more reliable.
But others possibly believe there is more to this, in any case; plus, ones who want something to be true can argue what they want to be true. So, I say God knows.
And my personal experience is the report about the adulteress is a fit spiritually with principles of how Jesus loved and related and did things, and His example for how He wants us to love.