Has anyone heard of "big" SEO companies removing links after termination of service?
Some companies own or have partnerships with large numbers of websites and place links out there for clients. They control these links and can modify or remove them easily - with programs or with low-cost workers.
The monthly fee for SEO services is often a payment for paid links. The links are actually "rented" by the month.
Do you think this is a legitimate concern?
Yes.
I think its physically possible to remove backlinks like this because it seems the SEO firm in question is building links by using other client's websites.
That is really sleezy to use the sites of other clients. My bet is that they have bought a large number of websites that owners have gotten tired of for a couple hundred dollars each. Its pretty easy to buy websites that owners have not touched in a long time. They might have also grabbed expiring domains.
I also wonder if they might have large content farm style sites where they place links for clients which might be quite easy to take down.
There you have it!