We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
this.eventBus._off("pagerendered", this._onAfterDraw);
      this._onAfterDraw = null;
      if (this.#onVisibilityChange) {
        document.removeEventListener(
          "visibilitychange",
          this.#onVisibilityChange
        );
        this.#onVisibilityChange = null;
      }
    };
    this.eventBus._on("pagerendered", this._onAfterDraw);
    
    
    Promise.all([firstPagePromise, permissionsPromise])
      .then(([firstPdfPage, permissions]) => {
        if (pdfDocument !== this.pdfDocument) {
          return; 
        }
        this._firstPageCapability.resolve(firstPdfPage);
        this._optionalContentConfigPromise = optionalContentConfigPromise;
        this.#initializePermissions(permissions);
        const viewerElement =
          this._scrollMode === ScrollMode.PAGE ? null : this.viewer;
        const scale = this.currentScale;
        const viewport = firstPdfPage.getViewport({
          scale: scale * PixelsPerInch.PDF_TO_CSS_UNITS,
        });
        const textLayerFactory =
          this.textLayerMode !== TextLayerMode.DISABLE && !isPureXfa
            ? this