I feel like they've made their position here quite clear and it's well-reasoned. I understand the disappointment, but this doesn't give any indications to me of "serial mishandling" unless there's some other context I'm missing?
Open source != open to contributions. Signal has made it pretty clear that their motivations for open source are visibility and verifiability, not to get people to do work for them for free. It seems like the action item to update the CONTRIBUTING.md to make those expectations more clear is a reasonable one.
They announced the feature coming to desktop "shortly" seven (7) years ago [1]. It has been implemented on mobile for ages, and is generally one of the most-popular features of any modern messaging service. In the years since, things like in-app cryptocurrency were implemented. Issues on GitHub dating back to 2017 were unceremoniously locked [2]. A community feature request has been open since 2018 [3]. When more issues on GitHub were created, they were told to discuss it in the one that was already locked, and didn't respond further once that was noted to be impossible [4].
When the PR was thoughtfully created long after it was clear that they wouldn't be honoring their own announcement, they said (approximately a year ago) that they would review and implement it with credit. After 6 months of darkness and petitioning, it was dismissed as being harder to review than to implement while disingenuously counting things like SVGs and license text as LOC. When some specific concerns were finally provided, the author responded point-by-point in how they were already researched and addressed, with a polite request for evidence so that they could correct any misunderstandings. The subsequent response ignored everything in that but the suggestion to update the contribution guide to align with their previously-unstated intent.
Serially mishandled. I'm not moving off of that position.
Open source != open to contributions. Signal has made it pretty clear that their motivations for open source are visibility and verifiability, not to get people to do work for them for free. It seems like the action item to update the CONTRIBUTING.md to make those expectations more clear is a reasonable one.