[Jimmy N]

I've had very good luck with Safety Seal plugs over the years, including with slashed sidewalls.

Be sure to get the Slim plugs if you go with Safety Seal as the regular ones can be tough to stuff through a small hole in the tread area of a radial.

 

[Jimmy N]

I think the record was 14 plugs in a slashed sidewall on a 35-inch tire.
It held air, but was relegated to a spare after that. A job it did for several years.

 

[Jimmy N]

Do you mean that it just held air or that it was run on occasion to replace a flat tire?

As I recall it never needed to be used after that incident.

It's so much quicker and easier to plug a tire than to put the spare on. Now, had another tire ended up with an even larger and unfixable gash, then that plugged one would've been Plan B.