Article 20 of the 27 October 1830 Draft Constitution of Belgium asserted the right to opinion: "The freedom of opinion in all matters is guaranteed."

In Belgium freedom of expression is guaranteed by Article 14 of the 1831 constitution: "The freedom of religions, their public exercise, as well as the liberty of expressing their opinions on every matter, are guaranteed; reserving the right of repressing crimes committed in the exercise of these liberties."

References:

English translation of the French text of the draft of the constitution of 27 October 1830 35 (2009): https://heinonline-org.proxygw.wrlc.org/HOL/Page?collection=cow&handle=hein.cow/zzbe0096&id=5&men_tab=srchresults

1831 Constitution of Belgium: https://www.constituteproject.org/constitution/Belgium_1831