It turns out that they do have the Winter Warmer at the 3 Brewers. It is their beer of the month for December (and beyond, I was told - it was rolled out later than planned so there is lots). At least this is the case on Yonge Street. On Adelaide Street the Harvest Ale remains the beer of the month. It appears that there is more variety between locations than I had suspected there would be. The Adelaide location has a different menu than Yonge Street's. Theirs matches the one that is found on-line.
The Winter Warmer is still wonderful but be wary: it remains 8% and it is available in pint glasses at a very good price, instead of the chalice style glass I am used to seeing. You could leave feeling very warm indeed.
So I guess the beer that I missed in December was the Holiday Ale.