Due to DIN 18105 - on the relationship between technology in standards and the law, the BGH decided in a judgment of 14.05.1998 – VII ZR 184/97 according to the sense and content in this case of DIN 820: The DIN standards are not legal norms, but private technical regulations with a recommendatory character. They can reflect the recognized rules of technology or fall short of them.
In safety-technical provisions in DIN standards there is a factual presumption that they are professionally correct, i.e. that they are “recognized rules of technology.” So now the question would be whether a conduit is safety-technically necessary because otherwise a danger would arise. I would clearly deny this (!) for telecommunications cables as opposed to elements required in the DIN such as surge protection or RCD. Thus, it is not an obligation for the electrician to use them—unless this has been expressly agreed.
So talk to the electrician and plan accordingly. But please note that you cannot use too tight radii and too many bends, otherwise you will not be able to replace anything afterwards anyway (for example with CAT-7 a minimum bending radius of around 50 mm). After three corners, such an additional empty junction box in the middle would also be quite sensible. Please use a diameter from DN 25, ideally even DN 32. That means, a DN 20 conduit installed at a right angle over 4 corners may also be a conduit, serves as a protective pipe—but only formally for exchangeability.