Generally, don't worry about whether a part/item is MIM or not.
If it bothers you, by all means, go ahead and change it.
This isn't to say a non-MIM part won't even break.
This is generally-speaking and not limited to 870 extractors.
Just my opinion.
Here's a non-MIM extractor (aftermarket) available, although I've no experience with this product or brand (Volquartsen):
https://www.midwestgunworks.com/page/mgwi/prod/vcrem
Actual Remington/Remarms parts seem to be hard-to-get or nonexistent right now (maybe forever), which is frustrating/disappointing.