I don’t know much about onion / orbot, but AFAIK orbot on android has some issues where some traffic could leak, so I won’t think using them on android for anonymity is a good choice. However my understanding could be outdated as I never really interested in using it. I know nothing about IOS though.
Although Telegram client is open sourced, but the main concern being all the data / metadata to supplied to Telegram, which unless you have excellent OPSEC, you could be tracked over time.
There are other points to think about whether you could safely use Telegram in first place.
- Telegram officially changed their policy in late 2024 saying they would cooperate with LE. For example, data of thousands of users had been handed to US
- Last month Chinese government demonstrated their ability to mass surveil Telegram Users, more explanation from a Telegram Channel could be found here (written in Chinese)
- Unless you use anonymous, deactivated phone number to hold the account, otherwise you could also be tracked if Telegram handed your user info to LE.
- Telegram is a buggy programme which enabled a lot of attacks in the past. Again, unless you have excellent OPSEC, using it to discover random topics, chat with random people is risky.
- If you use Telegram bots / business, it opens another attack factor.
I do have Telegram account and I still use it from time to time, but you really need to know what you are doing before using it.