Yes. You can still buy the season ticket now and get the BML once it releases. Edit: Even after BML comes out, the season ticket will still be there to buy.
Yeah, the season ticket is just a bundle of three routes. I can't see any reason that they'd remove it once the routes are all released, at least not until newer ones are close to releasing.
DTG have said in many streams that the season ticket will be available until at least the end of 2021.
Just to be aware - it's likely that the Season Ticket will remain on sale until the end of the year. It is however possible that it may stop being offered before that point. In the event that the season ticket will stop being sold before the end of the year we'll give plenty of notice.