Is it safe to eat this beef?

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Found some Beef Casserole Steak that has been in the freezer since December 2017, so 13 months, will it still be safe to eat? Will the quality have been effected much?

It’s probably OK, only just over a year old. But you won’t know for sure until you’ve defrosted it.
I’d certainly try it (and have eaten —much— older stuff from my freezer)

I'd try it too (but have a back up just in case it's not!)

will it still be safe to eat? Will the quality have been effected much?

Yes, it is safe. Given you're going to stew/casserole it, I doubt that it will be noticeably affected. It might be slightly dry and slightly less flavoursome.

Of course it is safe. By the time you have slow cooked it with plenty of flavourings, it will be delicious and I doubt you will even notice it's maturity!