The severity is easier to understand from the position of the community than from the offence.
The Portuguese Jews of Amsterdam were mostly descendants of forced converts who had lived outwardly as Catholics in Iberia and had returned to Jewish practice within living memory. They were tolerated in the Dutch Republic on terms that were generous by the standards of the age and were not guaranteed. The Reformed church watched them, and the city magistrates had made clear that the community was expected to keep its own order and to give no scandal to Christians. A member publicly denying the immortality of the soul was a danger to everyone.
The congregation also had recent experience of the problem. Uriel da Costa, who had denied the immortality of the soul and the authority of the oral law, was excommunicated twice, submitted to a humiliating public penance in 1640 and shot himself shortly afterwards. Juan de Prado was censured in the same period as Spinoza. Bans were a routine instrument of governance and dozens were issued, most of them short and revocable on submission.
This one was neither. Spinoza did not submit and did not seek readmission. He left Amsterdam, ground lenses, wrote, declined a chair at Heidelberg, and died in 1677. Proposals to lift the ban have been raised publicly in modern times; the atlas has not catalogued any act of the congregation lifting it, and records that as an open question rather than as a fact either way.