It's hard to say, if we're being genuine. By definition, maintenance mode means no new 'substantial' content. Obviously, Frontier is communicating that isn't the case. If we can take the team at its word, there ya go: no, it's not in maintenance mode.
Fact is, it's hard to take the team at its word much more. There's a great post titled something along the lines of 'Christmas is Cancelled, Again". This is a fact, too. Ice Planets and Fleet Carriers are just two features of the litany that haven't been delivered and were declared that they would be. We're essentially running into a crisis of credibility at this stage. It's not to Peter Molyneux levels, thank goodness, but it's bad. The bugs aren't killing Elite. The lies and lack of direction are.
Bug fixing is an ongoing process in most games and particularly online titles. The decision to fix the bugs is good, yes, but that ignores the fact they should have been a general focal point all along. Rather than promise content and then delay it continuously (which communicates you're incompetent, true or not), if bugs needed more time to fix at their introduction then the correct strategy is to say so and do so.
The game isn't in maintenance mode.
The credibility of the developer is, and has been for years, in maintenance mode.
This can very quickly lead to the game having no other route but to be forced into maintenance mode.
You have to fix the developer, first, before you fix the game they build.
Unfortunately, that's not something players/consumers can really influence as the sort of developer that is influenced by the clientele doesn't make the sort of mistakes that lead to posts like this, and so many others, in the first place. Perhaps the question to ask isn't, "Is Elite a lost cause?" and instead, "Is Frontier developing Elite a lost cause?"