We are often told by the railway industry that these major closures are planned far in advance and cant therefore be altered, presuming that this is true, why was almost no warning given by GTR of this closure.
There are going to be exceptional closures for emergency works at short notice. It feels at times though that short notice closures or effective closures (no trains running) have become anything but exceptional.
Depends where the engineering works are. If they are following up the temporary repairs put in place at Streatham (I think it was) that stopped trains to Victoria, then it was not planned months in advance.
The main line closure is Three Bridges to Brighton, hence a long bus journey, and I think it was listed on Southern's web site in May (when the problems with the Gatwick-Redhill buses were news). The March EAS lists it as Commissioning, Resignalling, and Maintenance. So no, it's not new - is there any evidence it wasn't easy to find out about if you actually needed to know?
What does seem to be missing from the advance information is a real alternative to the Journey planner. That's a good start if only one train interests you, and what they list for works by date then fills in the "how bad would it be" details. But for works that spread over many days/weekends, that doesn't help you with "I'd like to go to Brighton at least one weekend this summer - which day would be best?"
And how would you actually use this information, in practice, other than (as they do suggest too) not travelling at all?
Travelling to Brighton to / from stations between Three Bridges and London?
A limited train service will operate in both directions between Brighton and London Victoria via Littlehampton with journey times extended by 60 minutes, taking 2 hours 15 minutes between London and Brighton.
Customers travelling between London and Brighton are advised to use these services rather than the replacement buses.
These trains will only operate once an hour and space will be extremely limited. Not everyone will be able to board these services.
As it happens there is another long-planned possession at Streatham Common, but it's only on the line (Streatham Spurs) from Tulse Hill, so not likely to be useful.