There were some concerns regarding the strength of the frame when the 40 S&W models were introduced so Glock added the third pin. With one small exception, all Gen 5 guns are 9mm so apparently the strength of the frame isn’t a concern. The locking block on the Gen 5 guns is the same as earlier models, there is a provision for a third pin. It would probably be a simple matter to add a third pin if/when the Gen 5 comes in something other than 9mm and extra strength is needed.
The 9x19mm Glocks all lacked a Locking Block Pin until the Gen2.5 G26 was introduced in 1995. All non-9x19mm models except very early examples of some some models used the LBP.
In 2002, Glock added the LBP to the G17, and then a little later to all other 9x19mm models just to standardize Glock pistol construction. Then, with the introduction of the FBI-contract G17M and G19M (predecessor to Gen5 et al) in 2016, the LBP disappeared from subsequent 9x19mm pistols.
It is my opinion that the LBP has been shown by experience from 1982 to 2002 as unnecessary to Glock pistols in 380ACP and 9x19mm. I do not extend that opinion to Glock pistols in significant calibers regardless of a Brazilian police contract for a modified Gen5 pistol in 40SW or a submission of a Gen5 type of pistol in 40SW to the M17 MHS trials.
There was a change in the frame area near the locking block, and the locking block itself for the Gen 5. The locking block no longer rotates up slightly like it did in the early days, and that negates the need for the third pin.