Past-tense verb + that clause: clause has to use past tense?

I believe you can use either was or is but I would not use is in the sentence you wrote. This is a stylistic preference.

You state "Smith's work is always there and always thinks that the solution is feasible." Then why would you not think Smith et al. are not always present in [1]?

In [1], Smith et al. point out that the solution is feasible.

If it was some time ago, it might be better to stick with the past, and if you feel it is necessary to point out the continuing validity of the solution, you can do that.

In [1], Smith et al. pointed out that the solution was feasible, an option still in use today.

It partly depends on the (style and substance of the) rest of your paper, your preference, and the style preferences of the journal/audience to whom you will be presenting your work. I would either go with point/is or pointed/was. Pointed/is sounds a bit strange to my ear.