It's standard procedure for airlines to 'block out' the premium cabin/s (ie remove from sale all business and premium economy seats, even first suites if they have it) on an inaugural flight which is also a scheduled commercial service. This can be the first flight with new product, or a new route, or a new aircraft type.

Those seats are held back for execs, staff, VIPs, media, influencers, special invited guests etc, sometimes also any airline staffers who win a contest to be on the flight.

That said, I've been on inaugurals where only some of business class has been set aside (especially if the aircraft has a large cabin). But as a rule, airlines will block a slew seats or the entire cabin from the get-go based on expectations of how big an event will be and how many seats are likely to be needed.

As the date draws nearer and the list firms up, it's not uncommon to see some seats which are excess to needs released back into the publicly-available pool – so keeping an eye on availability, setting an ExpertFlyer seat alert etc is a good tactic if one knows when an inaugural may be.