As far as I know, they're taxed higher in the sense that they're not exempt from capital gains, like primary residences are, but in my view it seems like that would at-most be a marginal disincentive if renters don't have much in the way of other options or protections (which vary across the country). Likewise, I'm fairly sure a mom+pop residential landlord can save on tax for any expenses related to secondary suites or secondary properties if it's considered business income. If there is very little suitable supply, it still makes sense to acquire the asset and profit off of rent, although this dynamic may be changing with increased cost of borrowing and less demand depending on market, AirBnB short-term rental restrictions, etc..

When money was free and restrictions were few, non-institutional investors seem to have capitalized aggressively on this, because the price of the property increased dramatically and also it was a near-certainty that there would be a constant stream of inflated rent a landlord could charge.

I'm happy about those tax disincentives, but they do strike me as playing around the margins; in normal economic circumstances, they should play around the margins, but that particular aspect of the economy is so embarrassingly tilted that it's really not funny anymore.

Here in Norway non-primary residences were taxed higher but not very much. They've changed that in recent years, so now you get taxed about 100%. However here it's not (just) capital gains, it's wealth, so you pay a percentage of what the estimated street price is for the place each year.

The result is that in the last couple of years landlords have gotten rid of many rental units. This primarily affected smaller apartments, and surprise surprise, those prices have remained flat while the rest has increased.

On the other hand, sucks if you for some reason need to rent, because now there's just a few rental units out there and they're now expensive.