I don't think calling it slavery is accurate, but indentured servitude is close (except I don't think they experience maltreatment or harsh conditions).
Most white immigrants arrived in Colonial America as indentured servants, usually as young men and women from Britain or Germany, under the age of 21. Typically, the father of a teenager would sign the legal papers, and work out an arrangement with a ship captain, who would not charge the father any money.[10] The captain would transport the indentured servants to the American colonies, and sell their legal papers to someone who needed workers. At the end of the indenture, the young person was given a new suit of clothes and was free to leave. Many immediately set out to begin their own farms, while others used their newly acquired skills to pursue a trade.
The question of legality would come down to whether it's voluntary or involuntary servitude. But as a practical matter, I think it would be good for all people (not companies) if we found a more lenient path to citizenship as it wouldn't put as much downward pressure on salaries and wouldn't make H1-Bs preferable over others, all else being equal.
Your point actually makes H1B worse than "indentured servitude" of the past. Many (if not most) H1B folks stick around at one company just waiting for a green card. This kind-of means it is involuntary in some sense. However, unlike indentured servitude, there the "young person" does not automatically get a green card and is not "free to leave" at the end by default. By default, at the end, he/she has to leave the US.
Anyhow, this article is great:
https://en.wikipedia.org/wiki/Indentured_servant
One of the similarities that stuck out to me:
Most white immigrants arrived in Colonial America as indentured servants, usually as young men and women from Britain or Germany, under the age of 21. Typically, the father of a teenager would sign the legal papers, and work out an arrangement with a ship captain, who would not charge the father any money.[10] The captain would transport the indentured servants to the American colonies, and sell their legal papers to someone who needed workers. At the end of the indenture, the young person was given a new suit of clothes and was free to leave. Many immediately set out to begin their own farms, while others used their newly acquired skills to pursue a trade.
The question of legality would come down to whether it's voluntary or involuntary servitude. But as a practical matter, I think it would be good for all people (not companies) if we found a more lenient path to citizenship as it wouldn't put as much downward pressure on salaries and wouldn't make H1-Bs preferable over others, all else being equal.