Should all button text start with a verb?

I would say it depends on the action of the button. When you are dealing with Calls to Action (CTA’s) your primary emphasis is on communicating an action to an user as highlighted by this article about writing effective CTA’s which has this to say :

The very first piece of advice in the classic book Writing Tools
encourages writers to start sentences with subjects and verbs. In the
English language, we read from left to right, and verbs and subjects
help us to quickly glean the meaning of a sentence
. As basic as these
facts may be, acknowledging this when crafting your online
call-to-action is crucial when the attention of readers is more
elusive than the shadow of a flapping bird.

By not including a verb in the CTA copy, you aren’t prompting readers
to take action, which can hurt the click-through rate of your
call-to-action and negatively impact conversions.

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In fact, verbs are the part of
speech that generate the most shares on Twitter, which HubSpot Social
Media Scientists Dan Zarrella reported in his Science of Social Media
research. It turns out that verbs beat adverbs, adjectives, and nouns
in terms of their potential to attract Twitter shares!

I also recommend looking at this article for additional inputs on why CTA’s should be direct and should start with a verb

Be Direct: Your website is not the place to beat around the bush. It is imperative that you let your visitors know exactly what you do
and exactly what you would like them to do. Visitors shouldn’t have to
think about what to do next; be clear with your directives: “sign up,”
“download,” “submit,” “get started.”

Be Demanding: Your CTA should always have an action (duh). But make sure your action is short, simple and strong, to the point of
being demanding. Say exactly what you want the potential customer to
do: “download,” “watch,” “buy.” Avoid weak words that every other
website out there is using, such as “click,” “get,” “see” or “try.”

However if you are dealing with buttons which are just affirmative in nature, then this is what microsoft has to say :

  • Start labels with an imperative verb and clearly describe the action that the button performs. Don’t use ending punctuation.

Exception: The following standard labels are acceptable without verbs: Advanced, Back, Details, Forward, Less, More, New, Next, No,
OK, Options, Previous, Properties, Settings, and Yes.