According to LinkedIn they have 141 employees, down 30% in 2 years. We had nearly 400 at peak I think. They had a significant layoff in 2020.
I would not recommend using it unless Salesforce turns things around and starts investing back into the platform. If you are currently on it you should be making plans to move off of it. Ask your sales rep about "Project Periwinkle", which is intended to spin down Heroku and just make it all various Salesforce services. "Periwinkle" being a mix of purple and blue.
EDIT: I want to add that the common trope of "Salesforce killed Heroku" is not true—or at least it's not the full picture. Salesforce bought Heroku in 2010, only 3 years after it was founded. Salesforce largely built Heroku and later suffocated it through not giving us any headcount. There was a time when they cared, and if you ever saw our old office you know what I mean. I have no idea why Salesforce gave up, but us ex-Herokai debate about it a lot.