SAP BTP is the new wave of SAP services which are running in the Cloud managed by SAP.
The technology involved in these services is in many cases a further development of a Netweaver stack or HANA database. They are not always cloud native developed.
While you are used that Google, Amazon and Microsoft cloud services are (almost) always available, SAP BTP has some steps to make. It already improved the last few year.
For the services you want to use the planned SLA maintenance windows are published by SAP here:
Maintenance Windows and Major Upgrade Windows for SAP Cloud Services
As example:
The product HANA cloud has Zero down-time maintenance and up to 4 times per year max 4 hours outage.
The product “SAP HANA service for SAP Business Technology Platform, enterprise edition” has each maintenance window of 4 hours.
Other example: ABAP stack;
Up to 24 hours maintenance 4 times per year.
These outages planned by SAP cannot be stopped (nor should they be). It is your job to check with your company and the business process to be supported, if you can deal with the planned outages. If your availability needs are lower then using SAP BTP is fine. If you needs are higher than the SLA from SAP, you need to reconsider.