Is water softening useful?

My daughter is currently renovating the house of my deceased parents. After water, sewage, and heating are newly installed, the question of water softening also arises. The water has 21.4 °dH. The water pipes are plastic composite pipes. From acquaintances and at work, I have experienced that softened water can no longer be drunk pure. It tastes soapy and is inedible. But that would not be a problem. For the kitchen faucet, we would install a line with hard water. For the dishwasher, of course, soft water again. However, other problems are heard of. At Grünbeck, one hears of yellow deposits. Brass, and that is after all what the fittings are made of, is also attacked. The question now is whether the disadvantages outweigh the advantages or vice versa. On the one hand, costs in purchase, maintenance, and consumption (water and salt), possibly attacked fittings, anything else? On the other hand, fewer deposits in the storage tank, in the pipes, in machines, and easier cleaning. We all have a problem with customer services. If at all, such a system should be maintenance-free or self-serviceable. Honestly, forgetting the often said sales pitches. Is softening sensible or not? Would you install it yourself privately? What are the experiences? If yes, which brand is inexpensive and as maintenance-free as possible? Installation is not a problem.

There is already a very long topic about water softening here. The idea that you cannot drink softened water is nonsense; the system was probably set up incorrectly. Our Grünbeck does not/makes no deposits either. I am glad that we have it; my scalp also thanks me.

that softened water can no longer be drunk pure

Yes, it can. It must be possible as well, because at the tap point the water must comply with the Drinking Water Ordinance, regardless of whether it is softened or not. However, one often can taste a difference between softened water and tap water. Whether that is "better or worse" strongly depends on the individual case and the test subjects.

It tastes soapy and is undrinkable.

See above. With DIN-DVGW systems, there is usually no problem due to regular disinfection of the resin bed. Cheap systems from the internet, of course, do this rather rarely.

With Grünbeck, you hear about yellow deposits.

Those are (or rather were) isolated cases, not specific to Grünbeck and not due to the system technology. Almost always caused by improper installation or hygienically unsatisfactory production.

Brass, and that is after all what the fittings are made of, is also attacked.

Well, the way you put it here, the matter is not that general. At 8 dH, the likelihood of dezincification is very, very low.

Fittings and devices will thank you for durability if you use a softening system, and the shower experience will also improve. The taste of the water changes, that's true. It is not due to the "setting", but rather how many degrees of hardness you reduce, and at 21, that is a lot. We have 26, and there simply very many calcium ions are exchanged for sodium ions. We prefer to drink the unsoftened water and therefore have a separate faucet as a bypass. I think your considerations are good.

Is it possible to retrofit such a system, or is installation during new pipe laying/construction advisable for a specific reason?

Retrofit is possible without any problems. Certainly, it is more advantageous in new constructions from the very beginning. But this applies to all system technology in the house.