They are in the sense that you must pay tax on your rental payments.
Each country has different rules regarding the taxation of rental properties. In the Czech Republic, for example, if renting properties is your livelihood, i.e., you are renting for profit on the basis of a trade license (you are an entrepreneur), it is considered income from a trade and that is taxed under Section 7 of Act No. 586/1992 on Income Tax, as amended.
And, also in the Czech Republic, if you rent on a long-term basis, then the rented property is not a business asset, but it is considered rental income and is taxed under Section 9 of the aforementioned Act - the vast majority of Flatio leases fall into this category.
You can see the tax situations detailed here.