Many people are still working on improving RNNs, mostly in academia. Examples off the top of my head:
* RWKV: https://arxiv.org/abs/2006.16236 / https://arxiv.org/abs/2404.05892 https://arxiv.org/abs/2305.13048
* Linear attention: https://arxiv.org/abs/2503.14456
* State space models: https://arxiv.org/abs/2312.00752 / https://arxiv.org/abs/2405.21060
* Linear RNNs: https://arxiv.org/abs/2410.01201
Industry OTOH has gone all-in on Transformers.