I believe the platform closed at resignalling in 1992.
It was numbered as Platform 4, and it was taken out of use with the resignalling on 23 February 1992.
Cheers to you both for providing those dates.
I don't think the current Platform 3 has any booked passenger calls?
No, but until relatively recently was used by terminating/orginatinh Springburn services which had been introduced around the time of the Commonwealth games, and remained in order to provide Dumbarton Central with a 6tph service.
It can be used during disruption though. The removal of the opposite loop was fairly short sighted as it provides some useful capacity during disruption.
Those services were, at one time fairly recently, extended by four stops either end (Dalmuir-Dumbarton Central & Springburn-Cumbernauld); the latter section when the Springburn-Cumbernauld via Gartcosh section was electrified in May 2014. These services latterly took over the Airdrie-Balloch services' stops at Kilpatrick and Bowling during this period, the latter services running non-stop between Dalmuir and Dumbarton East (IIRC; they may have ran non-stop to/from Central).
In the end, this service was cut back to it's original terminus at Springburn when the Cumbernauld - Falkirk Grahamston section was electrified in December 2018. Can't recall precisely when the Dumbarton Central-Dalmuir section was dropped - was most likely a victim of COVID - but I do clearly recall changing (read: dashing down and up the passageways like a scalded rat lol) onto a 318/320 waiting at Platform 3 with such a service from a ex-Helensburgh 334 sometime in the latter half of 2019.
Given the overall track rationalisation across the entire Glasgow/Strathclyde area under BR and the PTE, I have always felt that Dumbarton was well down the list of any short sightednesses.
I would have thought so too, but given that Craigendoran - Helensburgh Central, Dalreoch - Balloch and Westerton - Milngavie all were rationalized in one way or another, I'm not surprised Dumbarton didn't escape completely unscathed. But as you say, it's usefulness was hamstrung by the lack of facing/trailing crossovers allowing trains from either direction to use it in the event of disruption, the surviving Down Loop on the current Platform 3 being far more versatile in this respect.
In the current more expansive environment do Transport Scotland have any plans to fund reinstatement of the loop?
I don't think any such plans for reinstating the eastbound loop/platform 4 at Dumbarton Central are or ever have been seriously proposed by Transport Scotland. On a hypothetical wishlist of infrastructure improvements/upgrades for the North Clyde Line west of Dalmuir, the full redoubling of both Craigendoran - Helensburgh Central & Dalreoch - Balloch sections would likely place quite a bit higher. That's not to say that it's a completely bad idea, as it would add a welcome bit of resilience to the network both generally and in the event of severe disruption.