@hugovk, @narendra.rajwar27, you're right. Drupal 8.7 is EOL and I would love it if the module takes a stance and says it won't support. But that should be for more meaningful code where there is value delivered (in terms of new API or removing deprecated API).
It's not worth to break support for removing a harmless line of code. Imagine someone still on Drupal 8.7 or before. This module will suddenly disappear from their extension list just because we didn't remove this line. On the other hand, keeping the line in has no cost to us or to users. It could be removed when Drupal 8.7 is a distant memory.
Last point: The same patch has been submitted by Project updates bot in #3141753: Automated Drupal Rector fixes. You'll notice there that it only adds the new line, doesn't remove the `core: 8.x`.
Thanks for your comments. I am not interested in turning this into a bikeshedding discussion over `core: 8.x` line. I'll let the maintainers decide. I just hope either of these patches is committed soon.