I searched various places like wikitech.wikimedia.org, mediwiki.org and github.com, but unfortunately couldn't find any up to date information about the state and further development of the Mathoid service. Therefore I am asking here.
Is there any roadmap available that gives insight into the future of Mathoid?
Not really. @Pchelolo and I take care of the development and maintain it and fix bugs if there are any. At the moment I am not aware of any major problems.
Are there any plans to make it compatible with more recent versions of NodeJS?
Maybe you can file a ticket for that. Is it incompatible.
Or will it be replaced at some point?
Once there will be MathML support for Chrome, we might replace it by native tex to MathML conversion in PHP to be developed. However, this is unlikely to happen in the near future.
Please reopen the ticket if you have further questions, or if you want to share additional insights with us. Also it might make sense to create several more specific tickets.