I hope I'm not making those - it's not my intention
Not if you have a (flexi) season ticket
I think this represents my view.
I wouldn't mind so much if the wording was "reservations full" or the ilk, and if there was a way for you to book the flexible ticket anyway on the understanding you'd probably need to stand, and there's a chance you might be unable to board due to crowding/safety.
But describing it as "sold out" is slightly disengenous, especially if the 1st class fares are sold as the alternative and the unwary are 'upsold' a 1st flexible fare without realising the standard class flexible ticket is still valid, if you know how to circumvent the journey planner.