Doxycycline is an extremely common antibiotic, but longer term use is not a common practice, except when treating malaria.
Is it still used long term for rosacea?
The dose of doxycycline usually prescribed is actually not high enough to even have any antibiotic activity. Since the antibiotics are prescribed at such low doses there are minimal long-term side effects. When using low doses of antibiotics at anti-inflammatory doses, studies have shown long-term use to be safe.
https://www.healthgrades.com/right-care/rosacea/is-long-term-antibiotic-use-for-rosacea-safe