Does the Scottish Heritage pass still exist?

Yes, it is. Historic Environment Scotland (HES) pass. Covers all the big castles - Stirling and Edinburgh, and lots of others too.
https://www.historicenvironment.scot/
We purchased joint membership for a year - best option because we were going to visit enough sites over a month to make it worth while. Also check out the Scottish National Trust. We also bought a joint year membership (covers site fees and parking fees). They manage the other sites not covered by HES.
https://www.nts.org.uk/
There is also a membership available that covers "stately homes", like Cawdor Castle, Dunrobin Castle, Inveraray Castle.
https://www.historichouses.org/visit-the-300-best-historic-houses-gardens-in-the-uk/the-best-places-to-visit-in-scotland/
depending on how many sites and the types of sites you want to visit, one or more of these memberships may be good for you.
Happy Travels
Heather

Yes, the Scottish Heritage pass still exists. You will have to determine if you are going to visit enough sites to justify the price of the pass.

https://www.visitscotland.com/wsimgs/Scottish%20Heritage%20Pass%20Booklet_1860044635.pdf

The link given by Mary just lists the attractions covered by the Scottish Heritage Pass. These include Historic Scotland properties, National Trust for Scotland and also Historic Houses. There is no date, no details of cost or details about buying it. Google isn't helping either, which would suggest this is no longer be available. Covering all three I think it would probably be horrendously expensive now too.

Decide which attractions you want to visit and who runs them. Work out how much admission will cost and if it is worth buying the individual passes.

Being thoroughly OCD, my spreadsheet showed I saved 50% with the Historic Scotland membership. Stirling and Edinburgh Castles alone were almost breakeven. I deleted the data after we got home last week.

Refer to the Historic Environment Scotland link previously referenced. The 2022/2023 membership booklet listing all the covered places is on the website. A complete list of membership categories and prices is also listed.

Thank you everyone! I think that wasleys is correct in that it has been morphed into another pass (or discontinued). I appreciate all the information about the other passes, though - that is very helpful and I will definitely be taking advantage of one or more of them once I figure out my itinerary.

I had made note of the Scottish Heritage pass when we were originally planning our trip in 2020, but just before we finally took the trip this May, I couldn't find a link to actually purchase it, so I believe it was discontinued. We did end up purchasing the Historic Scotland pass (which is good for a year and includes a magazine), because even though it didn't save us a ton of money on admission, it also gave us discounts at gift shops and restaurants at the sites we visited. It doesn't cover all the places that the Scottish Heritage pass did, but we did use it at Edinburgh Castle, Stirling Castle, Iona and Urquhart Castle.

The Explorer Pass was another one we looked at; it's cheaper than the Historic Scotland pass, but it's only valid for 7 consecutive days, and we couldn't make it fit with our itinerary. The HS pass ended up saving us more money in the long run.