I've seen a lot of people arguing that the mask orders are unconstitutional.

In Texas, at least, these orders are based on Texas Health and Safety Code chapter 81, which allows public health authority to issue orders and treats violations of those orders as Class B misdemeanors.

A lot of the conversation on this topic centers around Jacobson v. Massachusetts, 171 US 11 (1905) and an equally old line of quarantine cases.

I was curious whether there was any recent 5th circuit authority interpreting Jacobson in a modern light that might apply.

Sure enough, a case from April of this year. In re Abbott, 954 F.3d 772 (5th Cir 2020) - the abortion rights case where they dissolved a district court injunction on the Governor's COVID related abortion clinic shutdown. A major win for conservatives at the time.

In particular the court quoted with approval the standard set by Jacobson for when a court can review an order of this kind based on a rights claim:

"if a statute purporting to have been enacted to protect the public health, the public morals, or the public safety, has no real or substantial relation to those objects, or is, beyond all question, a plain, palpable invasion of rights secured by the fundamental law."

So is the mask order "arbitrary and oppressive" as the 5th circuit is going to require.

No. There is evidence that masks work, even if not everyone agrees with it, so it is not arbitrary. It is also less restrictive than the alternative - shutdown - so it isn't oppressive.

Interesting that the 5th Circuit's eagerness to hand a victory to the pro-life crowd is going to make any lawsuit about the masks an absolute loser, but life is full of little ironies.