And precisely because it's a niche language, and you have the experience now, you should definitely accept those legacy maintenance requests, as a freelancer, and ask for big bucks. You're wasting opportunities here.
Ada is/was heavily used in the defense/aerospace industry. Most of these legacy maintenance requests come from defense contractors who do not want to pay top-dollar for expertise. They want to put butts-in-seats to soak up billable time and keep the govt customer just content enough not to cancel their contract.
So they'll happily hire anyone with a clearance. If they can't find someone who already knows the language they'll eventually just hire someone who doesn't and let them figure it out on the job. Defense contractors do not pay big bucks for niche skills on legacy maintenance programs. Not to mention, it is some of the most boring work in the industry.
Let me tell you a story. 25+ years ago there was a bid in my city about how to renovate the main postal office. Architects all over the city did their bid, as requested by law, including the biggest of them all who had basically the entire local council in his pockets.
At the time I was a student, and was working as simple designer (AutoCAD) for an architect teacher who just opened his private architecture firm (6 months prior only). But the guy went to the board in question in last day, with a different bid. He proposed them bigger changes, not just renovation of exterior walls, but also design changes of interior space. Now doing that he made them go back to Ministry of Internal Affairs where the entire project instead of being billable as simple renovation, was changes and the entire budget would qualify for a superior plateau inside the Ministry levels of budgets. It jumped the qualification from (roughly estimated for today's $$$) $1M to $5M. And my boss at the time won. For next 6 months I worked my ass in AutoCAD to do all the changes and I am very proud of it even today, as it still stands as is currently.
Now here's your take of my above story. Get interested in their proposals, see what is about the projects and see if they can't actually be helped to jump to a bigger plateau of prices within government. You have the technical expertise, maybe it's time to create your relation expertise as well. They will jump through hoops and all if you bring them something to bill to something much higher their client (the defense department). Then you can really enjoy a bigger pay. Good luck and have fun.
Here is another example, when evaluating which language to pick for their security critical firmware, NVidia ended up picking Ada/SPARK.
At FOSDEM, Ada room has been a continuous presence for more than 10 years, or even longer, as the language still gets quite some use on high integrity computing around this side of the world.
