In linguistic terminology L1 and L2 refer to your native tongue versus second language, respectively.
So the process of declining performance in your native language is known as "L1 attrition", but it's an extremely under-researched topic. In case any academics are reading this that migth be aware of key papers, I'd appreciate a link or bibliographic reference.
A quick Google search for "L1 attrition file:pdf" or "first language attrition file:pdf" returns tons of results, so it doesn't seem to be that understudied. I think it mostly depends on what you want to focus on: do you want to know how a specific language or group of languages come to be lost by native speakers (e.g., indigenous languages)? Or are there some linguistics characteristics that you're more interested in analyzing (e.g., writing attrition, phonological attrition, grammar attrition)?
Here are some the things that I found; I can't guarantee they're all scientifically sound though, you'll have to do your own checks:
[2] Gallo et al., First Language Attrition: What It Is, What It Isn’t, And What It Can Be (December 23, 2019). Higher School of Economics Research Paper No. WP BRP 113/PSY/2019. https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3508640
It seems people are mixing active and passive skills as well here: For example, if you don't speak or write a language (L1 or L2) for a while, those activities will become more difficult but it doesn't mean you would lose listening and reading as well. OTOH if you are separated from your L1 community for decades, you'll have grown apart and there's potentially a huge gap culturally and in vocabularies.
Also, everyone has a smaller active and larger passive vocabulary, so alone it's not a sign of attrition.
So the process of declining performance in your native language is known as "L1 attrition", but it's an extremely under-researched topic. In case any academics are reading this that migth be aware of key papers, I'd appreciate a link or bibliographic reference.