Under the EU regulation there aren't really any "expiry dates" for car seats - legally you can continue to use them until the older regulation becomes unsupported.
Currently it's still legal to use seats conforming to R44/03, R44/04 and R129.
Personally I would not use an R44/03 seat - they are very old and outdated (approval between 1995-2008) and should be scrapped now. But the law says that you can.
If a backless booster seat was approved prior to 2017, then it is legal to use it from 15kg with no minimum age or height limit. It's only approval that matters, not manufacturing date, which means these boosters are still widely sold. It is not just older boosters which are kicking around in people's lofts/garages/car boot sales. For example, the Trunki Boostapak or Bubble Bum inflatable booster are both approved previous to 2017, so are subject to that minimum 15kg.
It is still possible to approve a backless booster under the R44/04 regulation (the only seat type which can be) although since 2017, all new approvals have that 22kg/125cm minimum. This will be printed somewhere on a sticker on the seat. Likewise any backless boosters approved under R129 will have the 22kg/125cm minimum.
The idea behind expiry is partially because of potential degradation in materials - most backless boosters are made of either polystyrene type foam with plastic or metal "horns" (belt guides) or they are made of hard plastic with a bit of foam padding. It's unlikely that hard plastic or metal will degrade to a point that they wouldn't perform. Polystyrene, I'm less sure of especially if it is the only thing holding the two belt guides together. It might be safer to dispose of a polystyrene booster after ~10 years (the estimated lifespan of a car seat). The ones which just have webbing or material attaching the lap belt guide to the seat should probably also be discarded after 10 years. If it has material belt guides rather than solid ones, avoid it altogether.
The other main reason behind expiry dates is that as time goes by, newer car seats on the market have newer safety features which either perform better or make the seat easier to use. Also, over time instruction guides get lost and stickers with instructions or safety warnings get worn away and difficult to read. Arguably this is all irrelevant with backless boosters - they are simple to use, and the ones on sale today don't really differ from the original ones for sale in the 1970s. The main difference today is that some have isofix which does afford a bit of extra stability, but that's not a huge deal.
There have been two instruction type changes - previous to R44.03, it was allowed to use a backless booster with a two point lap belt. Whereas nowadays all boosters need a 3-point seatbelt. And then the minimum weight change in 2017. Since a pre-2017 booster seat would now be nearly 8 years old at least, you might be able to find some information somewhere on the manufacturer's website recommending a max use period which will probably be something like 7-10 years, so you might have some luck with that. OTOH as said, there are still boosters being produced today with the 15kg minimum.
If this is to do with an ex insisting on a seat you're not comfortable with - using it in the back seat is much better than using it in the front, and middle seat will provide some passive side protection by taking the child further from any potential impact. While these seats are definitely not a great option for a child 15-22kg, one reason for the design not really changing since the 1970s is that the design actually works very well. As long as the lap belt is routed properly over the hips and belt guides, they are MUCH safer than being in a seatbelt alone (which is presumably what would happen without the law). And the design of some cheaper HBBs is so minimal that they are not really that much better than a backless booster anyway. The real problem is when children are in these seats much too young e.g. 2 or 3 years old, particularly if they fall asleep in the car (though the age is a problem alone) or are short enough that the shoulder belt is uncomfortable so they are told to tuck it under their arm or behind their back. Those are very dangerous configurations.
It's not hugely difficult to find info which strongly suggests that 125cm/22kg is the minimum even for existing boosters, or any sold/made post rule change - even though this is technically wrong, it might be something you can use to your advantage?