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
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:
[1] Schmid, M.S. 2011. Language Attrition. Cambridge: Cambridge University Press. https://www.cambridge.org/core/books/language-attrition/E01D...
[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
[3] Francis, 2023. When does second language learning lead to first language attrition? https://www.researchgate.net/publication/365372235_When_does...