It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements.
https://biblehub.com/acts/15-28.htm shows 27 translations, 22 of which uses the word seemed.
According to Google dictionary, the word seem carries less than 100% certainty:
used to make a statement or description of one's thoughts, feelings, or actions less assertive or forceful.
"I seem to remember giving you very precise instructions"
However, the reading of Acts 15:28 in its original Greek carries 100% certainty of determination from the Holy Spirit.
How about "it deemed good"?
Continuing with the assertiveness, the next verse is in imperative mood:
[You must] abstain
Verb - Present Infinitive Middle
Strong's Greek 568: To have in full, be far, it is enough. From apo and echo; to have out, i.e. Receive in full; to keep away, i.e. Be distant.
The verb appears in verses 22, 25, 28 and 34.
22 Then the apostles and elders, with the whole church, decided [ἔδοξε] to choose some of their own men and send them to Antioch with Paul and Barnabas. They chose Judas (called Barsabbas) and Silas, men who were leaders among the believers. 23 With them they sent the following letter:
The apostles and elders, your brothers,
To the Gentile believers in Antioch, Syria and Cilicia:
24 We have heard that some went out from us without our authorization and disturbed you, troubling your minds by what they said. 25 So we all agreed [ἔδοξεν] to choose some men and send them to you with our dear friends Barnabas and Paul— 26 men who have risked their lives for the name of our Lord Jesus Christ. 27 Therefore we are sending Judas and Silas to confirm by word of mouth what we are writing. 28 It seemed good [Ἔδοξενto] the Holy Spirit and to us not to burden you with anything beyond the following requirements: 29 You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things.
30 So the men were sent off and went down to Antioch, where they gathered the church together and delivered the letter. 31 The people read it and were glad for its encouraging message. 32 Judas and Silas, who themselves were prophets, said much to encourage and strengthen the believers. 33 After spending some time there, they were sent off by the believers with the blessing of peace to return to those who had sent them. 34 But it seemed good [ἔδοξε] to Silas to remain there.