I received beef suet from a grass fed cow. I have been storing the packages in my freezer. I was going to render some, and put 10lbs in my fridge to thaw some, and life happened and I forgot about it. That was 2-3 months ago. The packages are vacuum sealed. Are they still ok to render into tallow?
Most likely yes, if the suet was cut from around the kidney portion of the cow, which is very likely what you have if you live in any country where suet is an ingredient that many people use.
If it's not then you could have some connective tissue and such in there, so check for any kind of mold or a really offending smell. I've bought suet in Asia just to find out that it came from other (mostly short rib) trimmings, which didn't go catastrophically well.
So, 1 - make sure it's from around the kidney (or loin), and 2 - fat tends to keep amazingly well.