What do you mean? FOSS? Open hardware or able to integrate with existing foss like home assistant?

For the latter, I’ve used ecowitt based weather stations myself. they’re very durable, give reliable data and came with a quality manual for not much money.

They come with an app but you can just decide not to use it, all the data is available on the device, hosted as a web server. It has good integrations with HA which is what I use for long term storage together with influxdb.