Okay to use new KMC master link on old KMC 11spd chain?
Many master links from several vendors are specifically designated to be single use meaning you replace it with a new one after you unlock the old one. Which means you'll generally be fine by using a new master link.

For a moderately worn chain, adding a new link to the group of old ones means that less than 1% of your links will be a bit different (a typical geared bike has 110-116 links in a chain). It is unlikely that you will notice it.

If your chain is extremely old and stretched far beyond normal, adding a single shorter link to the mix may become noticeable, at least as a periodic sound appearing when this new link interacts with the rest of the worn drivetrain parts. In the worst case, it won't last because of that mismatch with the rest of the drivetrain.