Is 'But' an introductory word? [duplicate]

No, no comma is needed. Countless examples can be found showing no comma use.

(ref. 1, 1997) But is this true ? Is human health conserved by mandating ever - lower exposures ?

(ref. 2, 2014)But Will He Heal My Broken Heart?

(ref. 3, 2003) But I learned this weekend ~ it's not my life people would benefit from, rather God's work in my life.

(ref. 4, 2013) But she really couldn't accuse him of more meaningless sweet-talk, because he had no way of knowing about all that.

(ref. 5) But just imagine, what if that could be the solution for someone who was watching their loved one aging and dying before his eyes.

If you go through the whole page from which those references are taken, you will find that the use of the comma is exceptional.

(ref.) But, spiritual clothing, as you already know, must be worn one on top of the other.

It can be necessary sometimes when it is necessary from the point of view of syntax.

• But for two pictures that weren't framed the whole set had now new golden frames. (except for…)

• But, for two pictures nothing could be done, no restauration was possible. (however …)